Apparatus for forming a pre-applied underfill adhesive layer for semiconductor wafer level chip-scale packages

ABSTRACT

An apparatus and method for enhancing the formation of fillets around the periphery of assembled wafer-level chip scale packages when mounted onto substrates. The method includes fabricating a plurality of integrated circuit die on a first surface of a semiconductor wafer, each of the integrated circuit die being separated by scribe lines on the wafer. Once the circuitry has been fabricated, grooves are formed along the scribe lines on the first surface of the semiconductor wafer. The first surface of the semiconductor wafer is then covered with a layer of underfill material, including within the grooves formed along the scribe lines on the first surface of the semiconductor wafer. After the wafer is singulated, the resulting die includes a first top surface and a second bottom surface and four side surfaces. Integrated circuitry is formed on the first surface of the die. Recess regions created by cutting the grooves are formed on all four side surfaces of the die and filled with the underfill material. When the die is mounted to a substrate, the additional underfill material in the recess regions helps form more robust fillets than otherwise possible.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional application of prior application Ser.No. 10/366,067, filed Feb. 12, 2003 from which priority under 35 U.S.C.§120 is claimed, and which is incorporated herein by reference in itsentirety.

FIELD OF THE INVENTION

The present invention relates generally to a wafer level packagingprocess for integrated circuits. More particularly, the inventionrelates to apparatus and methods for enhancing the formation of filletsaround the periphery of assembled wafer-level packages mounted tosubstrates.

BACKGROUND OF THE INVENTION

There are a number of conventional processes for packaging integratedcircuits. One approach which is commonly referred to as “flip chip”packaging generally contemplates forming solder bump contacts (or othersuitable contacts) directly on an integrated circuit die on a wafer.After the contacts are formed, the dice are singulated by sawing orcutting the wafer along the scribe lines. The individual die can then be“flipped” and attached to a substrate such as a printed circuit board.That is, the solder bumps on the die are aligned and mounted ontomatching contacts on the substrate. The solder bumps are then reflowedto electro-mechanically connect the die to the substrate.

When a flip chip die is mounted to the substrate, an air gap typicallyremains between the die and substrate. This gap is commonly filled withmaterial that is flowed into the gap in liquid form and is thensolidified. This material is generally a mixture of a epoxy resin andsmall silica spheres and is often called underfill. A dispenser ornozzle is typically used to dispense the liquid underfill material atone edge of the die. The material then flows into the narrow gap due tocapillary action and spreads across the flip chip die until finally theentire area of the gap between the die and substrate is filled. Theunderfill material is then cured.

Since the silicon of the flip chip package and the substrate havedifferent coefficients of thermal expansion, the solder joints may failduring normal operation. The solidified or cured underfill materialhelps maintain the integrity of the solder joints, which in turn, helpsto reduce failure of the joints in the field.

As a general rule with wafer-level chip scale packages, it is desirablethat sufficient underfill material be present so that a fillet is formedaround the die when it is mounted onto a substrate, such as a printedcircuit board. During this process, the elevated temperatures causes thepartially cured underfill material to flow and to fully cure. Fillets ofunderfill material are formed as a result around the periphery of thewafer-level chip scale packages. Generally speaking, the more underfillmaterial present on the periphery of the die, the larger and more robustthe fillet. It is therefore desirable to have additional underfillmaterial at these locations. For more details on the benefits offillets, see “Effects of Underfill Fillet Configuration on Flip ChipPackaging Reliability”, by H. Nguyen et al., the InternationalElectronics Manufacturing Technology Symposium, SEMICON West, July,2002.

There are problems associated with aforementioned type of underfillprocess. For example, the operation of applying underfill must berepeated for each flip chip mounted onto a substrate. Repeating such anoperation many times during manufacturing significantly increases costs.

An apparatus and method for enhancing the formation of fillets aroundthe periphery of assembled wafer-level packages when mounted ontosubstrates is therefore needed.

SUMMARY OF THE INVENTION

To achieve the foregoing and other objects and in accordance with thepurpose of the present invention, an apparatus for enhancing theformation of fillets around the periphery of assembled wafer-level chipscale packages when mounted onto substrates is disclosed. The apparatusincludes a semiconductor wafer having a plurality of integrated circuitdice formed on a first surface of the wafer. The dice are separated fromone another by scribe lines. Grooves, filled with an underfill material,are formed along the scribe lines. The underfill material in the groovesincreases the amount of underfill material present at the periphery ofthe individual die after the wafer is described. When the chip ismounted to a substrate, the additional underfill material results in amore robust fillet being formed around the periphery of the die afterreflow and curing.

BRIEF DESCRIPTION OF THE DRAWINGS

The features of the present invention may best be understood byreference to the following description of the presently preferredembodiments together with the accompanying drawings. For the sake ofclarity the drawings are not to scale.

FIG. 1 (a) is a wafer with surface mount semiconductor dice fabricatedthereon.

FIG. 1 (b) is a surface mount semiconductor die scribed from the waferof FIG. 1( a).

FIG. 2 is a cross-section of the wafer of FIG. 1( a).

FIG. 3 is a cross section showing grooves cut along the scribe lines ofa wafer according to one embodiment of the present invention.

FIG. 4 illustrates a cross section of underfill material provided on thesurface and within the grooves of the wafer according to the presentinvention.

FIG. 5 illustrates a cross section of a die scribed from the waferaccording to the present invention.

FIG. 6 illustrates a cross section of the die scribed from the waferattached to a substrate according to the present invention.

FIG. 7 is an in-situ stencil used to apply underfill material to thewafer according to the present invention.

FIG. 8 illustrates the in-situ stencil on the wafer according to thepresent invention.

FIG. 9 illustrates a cross section of the wafer after underfill materialhas been applied to the wafer and the in-situ stencil has been removedfrom the wafer according to the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Referring to FIG. 1 (a), a wafer 10 with a plurality of integratedcircuit semiconductor dice 12 fabricated thereon is shown. Each die 12includes a plurality of solder balls 14 which are used to attach thesingulated die 12 to a substrate (not shown), such as a printed circuitboard. A plurality of horizontal and vertical scribe lines 16 (sometimesreferred to as “saw streets”) are provided to separate the individualdice 12 on the wafer. After the die 12 are fabricated, the individualdie 12 are singulated from the wafer by cutting or sawing the waferalong the scribe lines 16.

Referring to FIG. 1 (b) a singulated die 12 scribed from the wafer 10 isshown. The die 12 includes an active or first surface 17 where theintegrated circuitry (not shown) of the device is fabricated and thesolder balls 14 are formed. On the second or bottom surface, a backsideepoxy layer 18 is provided. The die 12 also includes four sides, each onthe periphery of the device. The type of die 12 illustrated in thisfigure is often referred to as a “wafer-level chip scale package” orsurface mount device because it can be flipped and mounted directly ontoa surface. For example when a printed circuit board is to be populated,the solder balls 14 are placed in contact and aligned with matingcontacts on the board. The board is then subject to a solder reflowprocedure in which elevated temperatures cause the solder of the balls14 to reflow, forming an electro-mechanical joint between the die 12 andboard.

Referring to FIG. 2, a cross-section of the wafer 10 is shown. From thisview, the solder balls 14 are visible on the top surface of the wafer 10and the backside epoxy layer 18 is shown on the bottom surface of thewafer 10. Cross sections of the scribe lines 16 are also visible on thetop surface of the wafer 10. It should be understood that only thosescribe lines 16 running in a direction perpendicular to the crosssection are visible. The scribe line 16 running in the directionparallel to the cross section are not visible in this figure.

As described in co-pending U.S. application Ser. No. 10/080,913,entitled “Method and Apparatus for Forming an Underfill Adhesive Layer”,by Nguyen et al., and assigned to the assignee of the present inventionand incorporated by reference herein, a technique of applying anadhesive underfill layer to the top or active surface 17 of the wafer 10is disclosed. In one embodiment of this application, the layer ofunderfill adhesive is dispensed and partially cured on the activesurface of the wafer 10. Once the underfill adhesive has partiallycured, the wafer is singulated. The individual wafer-level chip scalepackage devices or die 12 are then mounted onto a substrate such as aprinted circuit board. The solder balls 14 of the die 12 are then heatedto an elevated temperature (above the solder liquid temperature) duringthe reflow process. The elevated temperatures cause the solder balls 14to reflowed to form joints with corresponding contact pads on thesubstrate and the underfill adhesive to completely cure. In analternative embodiment, the underfill adhesive is fully cured after itis disposed onto the active surface 17 of the dice 12 on the wafer 10.In various other embodiments, the underflow adhesive is disposed ontothe wafer 10 using stencil printing, screen printing, molding, or aspin-on deposition process. The underfill adhesive is selected from agroup of materials including, but not limited to, epoxies, poly-imides,or silicone-polyamides copolymers and includes one or more of thefollowing components: epoxy resin, a hardener, a catalyst initiator, acoloring dye and an inorganic filler.

With the present invention, which is directed to apparatus and methodsfor enhancing the formation of fillets around the periphery of surfacemount flip chip packages when mounted onto substrates, theaforementioned process of applying the underfill layer is modified.Specifically, broad or wide grooves are formed in the wafer 10 along thehorizontal and vertical scribe lines 16. The underfill adhesive flowsinto and fills the grooves when it is applied to the active surface ofthe wafer using stencil printing, screen printing, molding, or a spin ondeposition process. In this manner, additional underfill material ispresent on the four peripheral sides of each die 12 after the adhesiveis either partially or fully cured and the wafer is singulated. In oneembodiment, a narrow dicing saw (relative to the width of the groove) isused to cut through and singulate the individual die 12. As a result, asis described below in more detail with respect to FIGS. 3 through 10,enhanced fillets are formed when the dice 12 are mounted to a substrate.

Referring to FIG. 3, a cross section of the wafer 10 showing grooves cutalong the scribe lines 16 according to the present invention is shown.The grooves 20 are formed by cutting or sawing the wafer 10 using astandard dicing saw having a blade that is wider than what is ordinarilyused for singulating the die 12 on the wafer. In various embodiments ofthe invention, the grooves have a width ranging from 1 to 4 mils and adepth ranging from 25% to 75% of the total thickness of the wafer 10. Inone specific embodiment, the grooves 20 have a width of 2.5 mils and adepth of approximately 200 microns with a wafer having a thickness ofapproximately 300 microns. It should be noted that these values areexemplary and in no way should be construed as limiting the invention.

Referring to FIG. 4, a cross section of wafer 10 with underfill material22 provided on the active surface 17 of the dice 12 and within thegrooves 20 is shown. When the underfill material is first applied to theactive surface of the wafer 10, it is in a non-cured or A-stage state.The underfill material therefore readily flows into the grooves 20formed in the wafer 10. Again, the underfill material 22 may be appliedusing stencil printing, screen printing, molding, or a spin ondeposition process, as described in the aforementioned co-pendingapplication.

Again, with regard to the cross sections of FIGS. 3 and 4, it should beunderstood that only those grooves 20 running in a directionperpendicular to the cross section are visible. The grooves 20 runningin the direction parallel to the cross section are not visible in thisfigure.

Referring to FIG. 5, a cross section of a die 12 scribed from the wafer10 according to the present invention is shown. The die 12 includessolder balls 14 formed on the active surface 17 of the die and the backlayer 18 formed on the second surface of the die 12. The die 12 alsoinclude four recess regions 24, each formed on each side of the die 12.The recess regions 24 are formed by the sawing operation described abovewhen the grooves 20 are cut along the scribe lines 16. The underfillmaterial 22 is shown filling the recess regions 24 and covering theactive surface 17 of the die 12.

FIG. 6 illustrates a cross section of the die 12 of FIG. 5 attached to asubstrate according to the present invention. During attachment, the dieis flipped so that the solder balls 14 are facing down and are alignedwith mating contacts on the substrate. The board then undergoes a solderreflow operation which causes the solder of the balls 14 to reflow,forming an electro-mechanical joint between the die 12 and the board.During the reflow operation, the elevated temperatures also causes theunderfill material 12 to reflow and cure, forming fillets 26 on all foursides of the die. Since the additional underfill material 22 within therecess regions 24 is substantially more than would otherwise be present,the quality and robustness of the resulting fillets 26 are superior thanpreviously obtained using prior techniques.

Referring to FIG. 7, an in-situ stencil used to apply underfill materialto the wafer 10 according to the present invention is shown. The stencil30 includes an opening 32 that has the same general shape as the wafer10. According to various embodiments of the invention, the in-situstencil is made of any polymeric or metal-based material that canwithstand the temperatures at which the underfill material 22 is cured.For example, the in-situ stencil may be made of a metal such as but notlimited to stainless steel, brass, any type of polymer, silicates,glass, etc.

Referring to FIG. 8, the in-situ stencil 30 provided on the wafer 10according to the present invention is shown. The opening 32 fits overthe wafer 10 so the underfill material 22 (represented by cross-hatchedlines) is prevented from flowing off the periphery of the wafer 10 whenit is applied. The in-situ stencil 30 is also designed to remain inplace on the wafer 10 while the underfill material 22 is cured on thewafer. This ensures that the underfill material 22 does not flow off thewafer before a full or partial curing. As a result, the underfillmaterial assumes a substantially uniform height across the entiresurface of the wafer 10. According to various embodiments of theinvention, the underflow material 22 may be either fully or partiallycured on the wafer 10.

Referring FIG. 9, a cross section view of the wafer after the underfillmaterial has been cured and the in-situ stencil has been removed isshown. As is evident from the figure, the underfill material 22 has auniform thickness across entire surface of the wafer and does not slopedownward at the periphery of the wafer.

In various embodiments of the invention, an underfill layer is appliedto the wafer 100 before the wafer is diced. The underfill may be appliedin a variety of different manners including, for example, stencilprinting, screen printing, molding or spin coating. In many embodiments,a B-stageable adhesive material (such as a B-stageable epoxy) is used toform the underfill material. Typically, the wafer 10 will have thesolder balls 14 before the underfill material 22 is applied. When aB-stageable underfill adhesive material is used, the underfill materialis either partially or fully cured after being applied. In either state,the wafer 10 can be readily handled and diced thereby singulating theindividual die 12. The resulting dice 12 can then be secured to anysuitable substrate (such as printed circuit boards, package substrates,etc.) using conventional solder reflow techniques. In situations wherethe underfill material 22 is only partially cured, the properties of theunderfill adhesive may be chosen so that the reflow process for theformation of the solder contacts during mounting finally cures theunderfill adhesive at the same time. In other embodiments of theinvention, a B-stageable underfill adhesive can be applied to the activesurface 17 of the dice 12 of the wafer 10 and then fully cured.

More specifically, after the solder balls 16 have been formed, a layerof underfill 22 is applied across the wafer 10. As noted, the underfilladhesive 22 may be applied using a wide variety of techniques includingstencil printing, screen printing, molding or spin-on processes. Eachtechnique for applying underfill has advantages and disadvantages. Byway of example, molding works well and uses readily available equipment.As described in the previously referenced U.S. Pat. No. 6,245,595, theballs 106 are typically (although not always) flattened in the moldingoperation which can be an advantage or a disadvantage depending on theapplication. Screen printing allows the application of variablethickness coatings using inexpensive tooling screens. Typically, whenscreen printing is used, a relatively low to medium solvent-based resinformulation may be used as the underfill material. Stencil printingtends to provide better height control than screen printing, althoughthe stencils tend to be more expensive than screens. As is well known tothose familiar with the art, stencils used in conventional stencilprinting operations typically have a relatively large opening in arelatively rigid sheet of material such as metal. The opening(s) is/areshaped to match that of the area(s) being printed. Otherwise, stencilprinting is quite similar to screen printing. Typically underfillmaterial with somewhat higher solvent percentages are used duringstencil printing than screen printing.

In one embodiment of the invention, the applicants recommend using anunderfill material 22 having the following properties: viscosity: 2,000to 20,000 mPa·s (milli Pascal·second); specific gravity: 1.0 to 1.4;solvent content: 20 to 45% (by weight); B-stage cure time of 20 to 30minutes at 100 to 150 degrees C. under vacuum; and filler content: 1-10%(by weight). Solvent is added mainly to control the viscosity of theformulation, which includes epoxy resin, hardener, initiators(catalysts), dye (for color), and inorganic fillers. Key desirableproperties of the underfill are: high glass transition temperature (Tg),low coefficient of thermal expansion (CTE), and good adhesion. High Tgmaterial allows the underfill material to go through high temperaturereflow with low risk of coating damage. High Tg materials are alsoobtainable through high molecular weight resins. Low CTE property isobtained through high filler loading. A preferred loading concentrationaims to produce materials with CTEs between that of silicon (3×10−6ppm/C) and the substrate (15×10−6 ppm/C) the die will be mounted on.Both options (high Tg and low CTE) tend to raise the viscosity of theformulation, and can be controlled by adding solvent.

By way of example, an underfill material 22 having a coefficient ofthermal expansion in the range of approximately 20×10−6/K toapproximately 30×10−6/K@25° C., typically works well in order to reducethermally induced stress. The coefficient of thermal expansion value oftypical solder balls 16 is also in this range. Close agreement betweenthe CTE values of these materials minimizes the generation of shearstresses between the underfill and the solder ball joints. One advantageto reducing thermal mis-match related stress is that the overallreliability of the electrical connection formed by the solder joint isgreatly enhanced.

In one specific embodiment, an underfill material 22 having a solventcontent of 40% is used. The observed advantages of using this percentageinclude: a lower viscosity so that flow coverage over the solder bumpsand wafer passivation is enhanced; lower potential for air entrapmentduring coating; a lower incidence of microscopic voiding trapped at thebase of the solder bumps; and an optimized B-stage curing profile. Toomuch solvent does not allow for proper flow.

With the use of an underfill 22 material with the above-definedcharacteristics, the applicants have found much of the solvent tends toevaporate during the curing process. Thus, the initial thickness of theunderfill layer 22 applied to the active surface of the wafer 10 needsto take into account the reduction in thickness due to solvent loss. Inthis example, the applicants have found that in order to produce a layerof underfill having a thickness that has a height substantially the sameas the solder balls 16, the pre-curing thickness of the material needsto be approximately 140% of the height of the solder balls 16. Invarious other embodiments of the invention, an underfill material with alower solvent content may be used. With the solvent content lower, theamount of solvent loss will be less. Therefore, the height of theunderfill layer before curing needs to be selected so that the finalcure height is at the desired level, typically at a height such that atleast the top surface of the solder balls 16 are exposed.

Although only a few embodiments of the present invention have beendescribed in detail, it should be understood that the present inventionmay be embodied in many other specific forms without departing from thespirit or scope of the invention. Therefore, the present examples are tobe considered as illustrative and not restrictive, and the invention isnot to be limited to the details given herein but may be modified withinthe scope of the appended claims.

1. A method comprising: fabricating a plurality of integrated circuitdice on a first surface of a semiconductor wafer, the dice separatedfrom one another on the first surface by scribe lines; forming groovesalong the scribe lines on the first surface of the semiconductor wafer,the grooves being sufficiently wide to form recess regions on the firstsurface in the dice; and providing a re-flowable underfill materialwithin the grooves and recess regions substantially across the firstsurface of the semiconductor wafer, the re-flowable underfill materialsubstantially filling the recess regions; and singulating the dice bycutting along the scribe lines, wherein the width of the cut is lessthan the width of the grooves such that some of the re-flowableunderfill material covers side portions of the singlated dice, andwherein sufficient underfill material remains on side portions of thesingulated dice after the singulation to enhance the formation offillets between the individual die and a substrate after the dice aresingulated from the wafer and mounted onto the substrate.
 2. The methodof claim 1, wherein the provided underfill material is a B-stageableadhesive that has at least one of the following properties: a viscosityof 2,000 to 20,000 milli-Pascals per second, a specific gravity of 1.0to 1.4, a solvent content ranging from 20% to 40% by weight, a B-stagecure time of 15 to 40 minutes at 100 to 150 degrees C. under vacuum, anda filler content of 1% to 10% by weight.
 3. The method of claim 1,further comprising forming a plurality of solder balls on each of thedice prior to providing the re-flowable underfill material within thegrooves and recess regions substantially across the first surface of thesemiconductor wafer, wherein the underfill material is provided at sucha height that the solder balls are at least partially exposed after theunderfill material is cured.
 4. The method of claim 1, furthercomprising at least partially curing the re-flowable underfill materialafter it has provided within the grooves and recess regionssubstantially across the first surface of the semiconductor wafer.
 5. Amethod as recited in claim 1 further comprising soldering a selected oneof the singulated dice to a substrate and reflowing the underfillmaterial to provide an underfill bond between the selected die and thesubstrate.
 6. A method, comprising: fabricating a plurality of dice on afirst surface of a semiconductor wafer, the dice being separated fromone another by scribe lines formed on the first surface of the wafer;forming recess regions in the plurality of dice along the plurality ofscribe lines on the first surface of the wafer; covering the firstsurface and filling the recess regions of the semiconductor wafer with are-flowable underfill material; and singulating the plurality of dicefrom the semiconductor wafer by separating the wafer along the scribelines on the wafer, wherein the width of separating cuts used tosingulate the dice is less than the width of the recess regions suchthat some of the re-flowable underfill material covers side portions ofthe singlated dice, and wherein sufficient underfill material remains onside portions of the singulated dice after the singulation to enhancethe formation of fillets between the individual die and a substrateafter the dice are singulated from the wafer and mounted onto thesubstrate.
 7. The method of claim 6, further comprising partially curingthe underfill material after covering the first surface and filling therecess regions of the semiconductor wafer with the re-flowable underfillmaterial.
 8. The method of claim 6, further comprising fully curing theunderfill material after covering the first surface and filling therecess regions of the semiconductor wafer with the re-flowable underfillmaterial.
 9. The method of claim 6, wherein the underfill material hasat least one of the following properties: a viscosity of 2,000 to 20,000milli-Pascals per second, a specific gravity of 1.0 to 1.4, a solventcontent ranging from 20% to 40% by weight, a B-stage cure time of 15 to40 minutes at 100 to 150 degrees C. under vacuum, and a filler contentof 1% to 10% by weight.
 10. A method comprising: fabricating a pluralityof dice on a first surface of a semiconductor wafer, the dice beingseparated from one another by scribe lines formed on the first surfaceof the wafer; forming recess regions in the plurality of dice along theplurality of scribe lines on the first surface of the wafer; coveringthe first surface and filling the recess regions of the semiconductorwafer with a re-flowable underfill material; singulating the pluralityof dice from the semiconductor wafer by separating the wafer along thescribe lines on the wafer such that the re-flowable underfill filledinto the recess regions results in the re-flowable material being atleast partially on all four sides of the singulated dice respectively;and forming fillets around the periphery of the singulated dice when thesingulated dice are mounted onto a substrate, the underfill material inthe recess regions on the periphery of the singulated dice used toenhance the formation of the fillets.